The 28-year-old Ivory Coast international complained that he felt unwell after training on Monday morning.
Following a series of thorough medical examinations conducted during the course of the day, a testicular tumour was ultimately discovered. Further examinations will take place in a specialist medical facility over the coming days.
“This news today came as a shock to Sebastien Haller and everyone else. The entire BVB family hopes that Sebastien makes a full recovery as soon as possible and that we can give him a hug again soon. We’ll do everything in our power to ensure that he receives the best-possible treatment,” BVB sporting director Sebastian Kehl said.